The present invention generally relates to a process for cleaning semiconductor wafers. More particularly, the present invention relates to a process for removing aluminum contamination that may be present on the surface of single crystal semiconductor wafers after the wafers have been etched.
Semiconductor wafers used in the fabrication of integrated circuits are produced by slicing thin wafers from a single crystal silicon ingot. After slicing, the wafers undergo a lapping process to give them a substantially uniform thickness. The wafers are then etched to remove damage and produce a smooth surface. The final step in a conventional semiconductor wafer shaping process is a polishing step to produce a highly reflective and damage-free surface on at least one face of the wafer. It is upon this polished face that integrated circuit fabrication takes place.
Following the lapping step the wafers must typically be cleaned to remove such things as lapping grit (e.g., alumina), organic residues, metallic contaminants, and other types of particulate impurities. If the cleaning processes are not effective, the surfaces of the wafer will be contaminated, or "stained," with these impurities. When integrated circuits are fabricated on a wafer surface stained with these impurities, the quality and performance of these circuits may be greatly diminished. To ensure high quality and performance, the wafers are typically inspected for stains by visually examining the wafers while under either bright light or fluorescent light illumination.
The contamination that stains semiconductor wafers typically originates from one of the wafer production steps. For example, in the lapping operation, the lapping grit comprises a large amount of aluminum oxide (alumina). The hardness and particle shape of the alumina make it particularly suited for the lapping application. A consequence of using alumina, however, is that aluminum contamination (in both ionic and particulate form) may adhere to the surface of the wafer. Furthermore, because the wafer is extremely rough after the lapping operation, aluminum contamination may become trapped in crevices on the surface of the wafer. Subsequent operations, such as chemical etching or cleaning processes, may not sufficiently remove this contamination. Furthermore, the use of heat in any of these subsequent operations amplifies the aluminum contamination problem by causing the particles to more strongly adhere to the surface of the wafer. The polishing operation may lessen the aluminum contamination on the wafer by removing several microns of the wafer surface, but the unpolished backside of the wafer remains contaminated. Failure to remove this backside contamination may result in staining that, in turn, could cause diminished performance in the final product.
Numerous processes have heretofore been proposed for reducing or removing the aluminum contamination that adheres to the surfaces of a silicon wafer after the wafer has been lapped. In general, however, these processes are not preferred because they fail to both cost-effectively and efficiently remove contaminants in an amount which is sufficient to prevent a reduction in integrated circuit performance and quality.
For example, it has been disclosed that directing ultrasonic energy through a cleaning bath effectively reduces the particulate concentration on the surface of a wafer (See, e.g., Erk et al., U.S. Pat. No. 5,593,505). Because of the expense involved with the use of ultrasonic energy, however, this type of process increases the cost of removing aluminum contamination from the wafer. Further, prolonged exposure to ultrasonic energy may result in damage to the crystal lattice of the wafer.
Another cleaning technique which has been used to remove aluminum contamination employs an oxidizing cleaning solution (e.g., SC-1, a solution comprising ammonium hydroxide, hydrogen peroxide, and water in a 1:1:5 ratio). Generally, this technique is not preferred because it makes the surface of the wafer hydrophilic (e.g. oxygen terminated). Aluminum oxide is known to react with the wafer surface when it is in this state, forming silicon-aluminum oxide. When the aluminum and silicon form this stable phase, it is extremely difficult to remove the aluminum from the surface of the wafer.
Thus, a need continues to exist for a process which inexpensively and efficiently removes aluminum contamination from the surface of an etched silicon wafer which does not require the use of ultrasonic energy.